ELECTKO\ MICROSCOPY 



Ol M. 



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Fig. 18. Patterns formed on lamellar gold crys- 

 tals due to diffraction effects. 



Fig. 17. Electron micrographs of thin colloidal particles of silver iodide showing 

 the presence of imperfections, a) mosaic crystal, b) dislocation lines and hexagonal 

 dislocation loop. 



ness of such crj\stals (33). Reasonable agree- 

 ment was obtained between the thickness of 

 particles obtained by this method and that 

 suggested by shadowing experiments. 



Examination of the Surface Structure 

 of Colloidal Particles by Replication. 

 Closely related to the problem of internal 

 structure of particles is the surface structure, 

 since the history of the mode of growth of a 

 particle is often recorded in its surface, in 

 the form of growth spirals, kink sites, twin 

 plane grooves, etc. Such information can 

 usually best be obtained by an examination 

 of very thin replicas of the particles shad- 

 owed very lightly with a heavy metal ^'apor 

 (42). However, in the case of very thin crys- 

 tals, composed mainly of elements of low 

 atomic weight, it is often possible to detect 

 details of surface structure by direct micros- 

 copy or by lightly shadowing the specimen 

 with a heavy metal vapor. Typical exam- 

 ples obtained with a metal detergent crystal, 

 barium tetradecyl sulfate, are shown in Fig. 

 19. The spiral terraces of the plate-like crys- 

 tals are clearly visible, the lengths of the 

 shadows indicating the depth of the steps to 

 be of the order of 50 A, i.e. bimolecular, and 

 indicate that growth probably occurs by a 

 screw dislocation mechanism. Similar effects 

 have been observed with single crystals of 

 the paraffin n-nonatriacontane and stearic 

 acid (43). In the latter case the crystals were 



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Fig. 19. Electron micrographs of barium tetra- 

 decyl sulphate crystals showing surface structure, 

 a) shadowed with chromium at 50°, b) direct 

 micrograph. 



particle. This fact has been utilized as a 

 method for the determination of the thick- 



140 



